I could use some help with a 50 year old Sona Amp. Has to be something simple.

[sluckey]

I urge you to tell the owner to take this amp to someone that is more knowledgeable.

I realize this sounds very sharp. Someone needed to say it. Wish I had more finesse but unfortunately I'm a very plain spoken person. It's a curse the Luckey family has to bear. Pity my wife!

[Paul1453]

I will check out that resistor once i get moving this morning.  and check voltages.  I was tining of building a limiter, saw one in a video.  Ill look for some plans for one.


Dave

If R19 or R20 are the problem all you need is a 330K resistor to replace the burnt one.  If that fixes the output issue you won't even need to check any voltages.  ;^)

[Paul1453]

I urge you to tell the owner to take this amp to someone that is more knowledgeable.

I realize this sounds very sharp. Someone needed to say it. Wish I had more finesse but unfortunately I'm a very plain spoken person. It's a curse the Luckey family has to bear. Pity my wife!

It's all good, sluckey!

I owe the forum some payback for all the good advice I have received.  I don't think this one requires one of you electronic engineer types to solve.  More like a Columbo type detective tech to help him on this one.   

[highlux]

Thanks

[Willabe]

I was tining of building a limiter, saw one in a video.  Ill look for some plans for one.

Here ya go, from Sluckey's web site;

http://home.comcast.net/~seluckey/amps/misc/Amp_Scrapbook.pdf

[sluckey]

3/4 preamp tubes are original.

Tubes are always the #1 suspect. They are easy to substitute and should always be confirmed or ruled out as the first step.

[Paul1453]

3/4 preamp tubes are original.

Tubes are always the #1 suspect. They are easy to substitute and should always be confirmed or ruled out as the first step.

I can agree with this to a point.  I like to follow my Army troubleshooting sayings.  KISS, Keep It Simple Stupid.  I always start by sniffing around, then looking around, and then poking around.  If I smell or see any burnt components, I like to replace them first before offering up any more tubes for sacrifice.

[highlux]

Thanks

[Paul1453]

I usually just take a three prong cord from an old computer or other non-functional equipment I have at home and cut the end off that plugs into the computer's power supply.   

[highlux]

Thanks

[Paul1453]

highlux,

Would you like me to explain my thought process on how I came to the conclusion that either R19 or R20 is causing the problem with this amp?     If I were fixing this amp myself, I would test and if found to be bad, repair that issue before I did anything else.   

[sluckey]

Would you like me to explain my thought process on how I came to the conclusion that either R19 or R20 is causing the problem with this amp?

I'd like to hear it.

[Paul1453]

Would you like me to explain my thought process on how I came to the conclusion that either R19 or R20 is causing the problem with this amp?

I'd like to hear it.

I'm just an Army trained repair tech, trying to learn some of what you experienced engineers know about designing/optimizing circuits.  So I stick to what they taught me to make the gear operational as quickly and efficiently as I possibly can.  1st rule is KISS, Keep It Simple Stupid.  Then for the Commo gear I was taught to repair, there is a hierarchy to follow PSTD, Power, Switches, Timing then Data.  I'd estimate that between 1/3 and 2/3rds of all the repairs I ever made were at step 1 Power.  As I mentioned earlier power problems tend to burn things, so without even turning a screw to open things up I can often just take a couple of sniffs around the gear and if I noticed that familiar burnt smell I know I'm probably dealing with a power issue.  Then I open it up and do a thorough visual inspection, even if I don't smell anything to indicate power problems.  I check all inputs/outputs and circuit cards looking for loose/broken wires/switches/components.  Paying special attention for burnt or heat stressed areas if I did smell something.  Then I move to poking around the components with a chopstick/plastic probe.  Seeing if wires are loose or simple fall off when poked.  I try to check underneath any particularly congested wiring areas looking for any metal parts like washers or nuts that may be just floating around in the gear.  Poking resistors and such can often reveal cold or broken solder joints.  Just by using this type of inspection I quite often find issues that need to be dealt with.  On highlux's amp I noticed that resistor that looked charred to me from the pictures.  I noted the color code on the resistor and calculated it to be 330K.  I then checked the schematic looking for 330K resistors.  The 1st place that caught my eye was R41 connecting B+2 to V5a.  I then noticed R28 connecting B+3 to V4b.  I then had to think about the symptom, low volume output, and which of these places would cause that.  R41 is in the tremolo circuit, and R28 in the reverb circuit.  I then thought if the tremolo or reverb circuits were bad how would this cause the output to be low.  I looked at both circuits and could not see how either of these resistors could cause low output.  So back to searching the schematic for 330K resistors.  Luckily this is not a huge schematic and the only other place I could find 330K resistors is just after the Phase Invertor and right in front of the Push/Pull output tubes.     Aha, I can easily see how if one of these (R19, R20) were open or burnt enough to be of considerably different value than the other that the output tubes might be fighting each other or just not able to produce expected level of output.  With the what looked to be a burnt resistor 1st discovered in the visual inspection, and a careful evaluation of the symptom and circuit these resistors are in, the 1st thing I would do is replace that burnt resistor.  If after that we power up the amp and the volume issue is resolved, well it's done.  If the symptom remains and that resistor I just replaced gets very hot again, then I would need to look more closely in that area for more issues.

Does that make sense to you?  It often helps me to try to explain my thought process to another experienced tech.

[Paul1453]

Maybe this thought will spur some comments.   

My sniff, do a thorough visual inspection, and then poke might also be considered by some as a plausible approach to dealing with women as well.   

[sluckey]

R19 and R20 are used to couple the tremolo signal to the grids of the output tubes. The trem signal varies the grid bias of the output tubes at a slow rate in order to produce the tremolo effect. You could totally cut R19 and/or R20 completely out of that circuit without affecting the amp. Well, except you will affect the trem if you only cut one resistor, or you will have no trem at all if you cut both resistors.

[Paul1453]

R19 and R20 are used to couple the tremolo signal to the grids of the output tubes. The trem signal varies the grid bias of the output tubes at a slow rate in order to produce the tremolo effect. You could totally cut R19 and/or R20 completely out of that circuit without affecting the amp. Well, except you will affect the trem if you only cut one resistor, or you will have no trem at all if you cut both resistors.

So where did my analysis break down?  I realize this is the insertion point for the tremolo signal, but wouldn't a significant mismatch in those resistor values possibly cause different signal levels to be felt on the grids of the PP output tube pair?  I would also expect that to be the case if R23 and R24 were significantly mismatched.   

Isn't this point also indirectly connected to the NFB loop from the secondary of the OT?

Maybe that resistor was just dirty and not burnt to begin with?   

[highlux]

Thanks

[sluckey]

Isn't this point also indirectly connected to the NFB loop from the secondary of the OT?

Not even close.

Maybe that resistor was just dirty and not burnt to begin with?

That's what I thought. But either way, it ain't causing the symptoms we've been given. Maybe highlux could check that resistor for you.

BTW, there's an error on that schematic. The junction of R36, R37, and R43 should have a ground symbol.

[Willabe]

BTW, there's an error on that schematic. The junction of R36, R37, and R43 should have a ground symbol.

Aha! I thought that was screwy. 

[PRR]

> if one of these (R19, R20) were open or burnt enough to be of considerably different value than the other that the output tubes might be fighting each other or just not able to produce expected level of output.

V2b outputs as 47K impedances.

The input of (say) V6 is 330k||1Meg, or 248K.

47K driving 248K is essentially 50K driving 250K, a light load. The 248K could be any larger value and not have much effect. (20% difference in push-pull drive is nearly no effect until you get out the fine test equipment.)

If the 330K went _down_, say to 50K, it would have some real effect. I have not looked at the pictures. But if you see "burnt", *usually* the value goes *high*. (And not always.)

> make the gear operational as quickly and efficiently as I possibly can

Some of us have been there. I know a guy here maintained airport flight guidance, and you can't tell a 747 to come back Tuesday. I did a lot of live PA, and the show must go on. I think someone here had the same training and duty as you.

My first semi-mentor said "Is it plugged in? Is it turned on?" To which I add "Is it really plugged-in and getting power, is signal really going where it's gotta go?" Obvious mistakes are rampant. Connectors have the worst job, yet are often low-cost.

And when it is really not-on, a sniff is part of the trip.

But in wide-open classic tube audio, voltage-checks are a very reasonable 2nd step in the process. It will find (or rule-out) about 2/3 of the parts (tube, resistors, shorted-caps). It isn't much work. For 85% of stages you don't really need a voltage-table. Grid at 0.0V (with exceptions). Cathode a few volts. Plate should be at 1/3rd to 2/3rd of its B+ node. (Though here, V2 is an exception.)


BTW: Sniffs can go wrong. I got a trashed power supply with a meter. Someone had slit the sleeve over the fuse to check it (OK). When I looked the other way, I saw a "toasted" thin wire on the back of the meter. Yup, toasted right through. And so thin and so long-ago that any smell was long gone. (For more fun: when the shunt let go, the tiny meter tried to take the full 2A load, and burst a hair-spring. You can't get these no more.)

I jumpered the burnt stuff, it worked, and then it didn't work. After much swearing, I discovered an assembly mistake. The voltmeter PCB hit a chassis screw (maybe they changed meter vendors). Someone had snipped the corner off the PCB so it would "clear". Well it did or it didn't depending how you set the upper/lower chassis halves. But because of the current-limit, there was no POW from the short, no stink, it just shut-down.

[PRR]

> Owner says the volume dropped one day and stayed low.

Do you believe him?

My eye sees that V2 *must* be a 12AU7. Unlike most tube stages which can take 12AX7 12AT7 12AU7 with acceptable but declining gain, the direct connection from V2a to V2b suggests a very critical circuit, which will shift to a non-working bias if a 12AT7 or 12AX7 is in that socket.

Voltage checks would be suggestive. Please do not get a shock!!

What are heaters?? These are vacuum tubes. Electricity will not go through a vacuum. Electrons can carry electricity through a vacuum, but electrons are tightly-bound in atoms. But if we heat atoms about white-hot some electrons are liberated. We get this heat any convenient way, which is invariably a "light bulb filament" and a power supply (DC or AC, generally low voltage, invariably 6.3VAC in classic tube amps).

But you probably do not need to check heater supply if the tubes light or get warm. (Say you are blind, but you want light for a friend who is coming. If the bulb gets warm/hot, you don't need a light-meter or a volt-meter to be pretty-sure it is working.)

6CS7 (reverb driver) is an odd old duck. Fortunately it is unlikely to be the immediate problem.

And yes, well-spotted: Rev and Trem Int pots go to a ground, also the stopper off the Trem Speed pot.

[Paul1453]

Isn't this point also indirectly connected to the NFB loop from the secondary of the OT?

Not even close.

Maybe that resistor was just dirty and not burnt to begin with?

That's what I thought. But either way, it ain't causing the symptoms we've been given. Maybe highlux could check that resistor for you.

BTW, there's an error on that schematic. The junction of R36, R37, and R43 should have a ground symbol.

That ground would eliminate that NFB connection I thought I was seeing, and with the clearer pictures posted it definitely does not look charred on one end as I thought it was.  Other than that, are there issues with the methods the Army taught me?  It has always served me well to fix any issues I uncover in my inspection prior to commencing troubleshooting.  Especially the charred and burnt components.  I always try to figure out what kind of failure could have caused the component to burn like that.  On a few rare occasions the replaced part has burned up again, when I haven't completely figured out the underlying cause yet, like a loose washer sliding around intermittently shorting things.   
Many times I have come to a problem where multiple higher ranking limited maintenance techs have been troubleshooting a problem for days and with a good inspection have found a problem replaced the part and fixed the gear without any troubleshooting required.  I became so familiar with one piece of gear that I would just go over and smell the power supply, and if I detected that all to familiar smell I would tell the lower level techs this one needs it's power transistors replaced first.

So on highlux's amp I guess I'm back to stage 1 Power.  Are the Voltages all correct?

[highlux]

Thanks

[sluckey]

Other than that, are there issues with the methods the Army taught me?

Nothing wrong with your look, sniff, poke training. But we are way beyond that. And this is old tube technology. That means change the tubes FIRST. WHY? Because history and experience have shown that tube failure is highly suspect. AND changing tubes can usually be accomplished without having to pull a chassis! Many pieces of tube equipment have been fixed without having to go any further than a simple tube change. TUBES MUST BE ELIMINATED AS A SUSPECT.

So on highlux's amp I guess I'm back to stage 1 Power.  Are the Voltages all correct?

And with that, we come full circle, back to reply #1! After almost 3 days and 72 replies, highlux has not tried known good tubes, nor has he provided the requested voltage measurements. Attempts to repair this amp over the net are FUTILE until he provides that info.

In the original post highlux said 'I dabble in some amp repair." After about 16 replies, I knew I had incorrectly ASSUMED that meant way more than it really did. That's what prompted my sharp statement that you didn't like.

[sluckey]

I got a magnifying glass and flashlight and was finally able read the numbers in the sockets. Tiny.

There's a much easier way to determine pin numbers. Notice that all the tube pins are equally spaced except there is a wide space between two pins. THAT'S THE KEY. The tube manufacturers did that so the tube pins will fit the correct socket holes. You can use that KEY to mentally count pin numbers. Looking at the bottom of the socket find the wide space. The first pin clockwise of that wide space is always pin 1. Counting clockwise, the next pin would be pin 2. Etc, etc, etc...

See pic...

[highlux]

Thanks

[plexi50]

There is no insulting going on or intended. It's just if you want to know what the problem is you must measure the voltages of the preamp tubes and tell us what they are. That tells the story of what is or may be wrong with the amp. It is easier to rule out and tackle one thing at a time.

Then on to other things like Bias and power supply resistor nodes,etc,etc,etc

[highlux]

There is no insulting going on or intended. It' just if you want to know what the problem is you must measure the voltages of the and tell us what they are. That tells the story of what is or may be wrong with the amp. It is easier to rule out and tackle on thing at a time.

Thanks to all that helped.

[EL34]

There is no insulting going on or intended. It's just if you want to know what the problem is you must measure the voltages of the preamp tubes and tell us what they are. That tells the story of what is or may be wrong with the amp. It is easier to rule out and tackle one thing at a time.

In order for the guys here to help, people need to post the amp voltages when they are requested to do so

It's one of the first thing that should be checked when diagnosing an amp

It's very frustrating for someone to try and help and never get the info they need to help.

[plexi50]

Please post your voltages. Is the amp fixed? In either case post your voltages for all of us to see.  It's history,documentation & specifics that drive many of us. And for others that will find your amp and thread in the future and have some reference to this amp.

[highlux]

Thanks

[plexi50]

Before you measure any voltages do so with the old tubes in the amp as it is now. If you put new tubes in the amp and then test voltages you are defeating the purpose of the process of elimination. No one is putting you down or treating you any different than what is expected in an answer to a question. The power cord is a good move being that bad.

[highlux]

Thanks for trying guys.

I decided that I didn't like being talked down to by a Moderator when I clearly stated I was an amateur.

I just wanted to fix this amp....for free....for a friend. I'm a good guy. I donate practice amps and guitars to needy kids....I never charge anyone for
repair work.

I would have powder coated enclosures for you guys...sand blasted old parts....I would have given my all to help.

Maybe I'm sensitive.  But I have a choice.  I am a good guy who would help anyone and never cut someone down or talk down to them.

I wont put up with it,  especially from a Moderator...no matter how knowledgeable...no reason to treat me like that....with the only possible reason or outcome of what
you said was to fluff yourself up.

What possible reason other than your own ego boosting would you discourage me from learning?   insinuating I am not capable of understanding your arcane tomes of knowledge only you possess. 

Goodbye and thanks for all the help.

[sluckey]

Wow.   Its going to be hard for me to be grateful if you keep subtly insulting me.

I would rather go on a 3 week journey with Paul and solve this issue, than have your help any longer.

You sir can excuse yourself from my thread. You are fired. No powder coating for you.

I will step aside. Not because you fired me but because you refuse to give us the info needed to help you.

If you are uncomfortable checking voltages or unsure about how to do it, just say so. Plenty of people will give you a hand. But they cannot spoon feed you. You gotta do some of this shit yourself!

Take a road trip. Pack a big suitcase because it's gonna be a long ride if you continue in this same direction.

[highlux]

And this is exactly why I left.


This guy is a global mod huh?  An ambassador to the site.  Wow.

[shooter]

And this is exactly why I left

I used a modified version of "you can lead a horse...." when I taught teens Sunday School, "you can lead 'em to water but God won't let you drowned them"

the collective knowledge here is by far the best you'll find in cyber-space, bar none.
If your friend lives in MI, be glad to work on his amp with the knowledge I've gained from here 

[highlux]

I didn't want to leave. But I have my pride.  No...Im in St. Louis area.  There are lots of places we could have taken it....but how am I going to learn?
Just like with the cnc machine I am ordering from shop bot. I have to learn solid works and some other software to be able to use it...but that wont stop me.

Even a noobie deserves common courtesy and respect.

[Ambugaton]

This escalated quickly... think about the thought process for a sec. Sluckey got a little agitated because you didn't provide voltage readings in order for him to be able to help you in a timely manner. I think that is something you can get over.

[highlux]

Why would I continue to endure negative comments?     In not so many words...he says mistake for me to fix it. Should have taken it to a pro.   That he expects it to fail.

I dont need that.   Lift me up. Encourage me. Teach me.  I came in humble....asking to learn. Literally said I would follow instructions and not talk back.  Thats coming in humbly.
As much as I know how to do.  I didn't deserve to have the wind sucked from my sails like that.  Sensitive? Maybe. I am in considerable pain physically.  But I made an effort to be nice...polite...humble...an appreciative.  Hell I am even giving. I was willing to powdercoat enclosures and stuff for people I never met.

To be stepped on was not what I expected....and It rubbed me the wrong way.

I really didn't expect it from a moderator.  Maybe a grumpy member...but not a leader. Makes me wonder if I am just fodder for a mod to feel superior.

I know I am the loser here...I lose out on a killer site. didnt want it to go that way....but then again...I really didnt do anything wrong, or warrant such treatment.

[Paul1453]

I'm starting to look at sluckey's comments as strong medicine.  It's often bitter and sometimes makes you feel worse to begin with, but I know I need to endure it and let it soak in to help me get better in the long run.   

[Ed_Chambley]

Why would I continue to endure negative comments?     In not so many words...he says mistake for me to fix it. Should have taken it to a pro.   That he expects it to fail.

I dont need that.   Lift me up. Encourage me. Teach me.  I came in humble....asking to learn. Literally said I would follow instructions and not talk back.  Thats coming in humbly.
As much as I know how to do.  I didn't deserve to have the wind sucked from my sails like that.  Sensitive? Maybe. I am in considerable pain physically.  But I made an effort to be nice...polite...humble...an appreciative.  Hell I am even giving. I was willing to powdercoat enclosures and stuff for people I never met.

To be stepped on was not what I expected....and It rubbed me the wrong way.

I really didn't expect it from a moderator.  Maybe a grumpy member...but not a leader. Makes me wonder if I am just fodder for a mod to feel superior.

I know I am the loser here...I lose out on a killer site. didnt want it to go that way....but then again...I really didnt do anything wrong, or warrant such treatment.

I am putting this on the forum instead of a PM for anyone who has felt talked down to.
It is really your call if you are gone, and I do understand.  When I first came to this forum I was good with old Radios and could cobble together a kit and do a few mods.

So let me tell you what I know about the moderator that upset you.  Man has that guy pissed me off before and I have pissed him off a time or 2.  Then came a time where I became interested in an old design of a AC15 that he and another member had been kicking around, but never built.  I had a 63 AC15 when I was younger and loved the amp but in those days I could no afford to keep an old Vox running and sold it.

Well a guy decided to build it at another forum and we invited him to share his build with us.  I was really wanting to build it bad, but I wanted to change a couple of things with the layout.  Sluckey had shared the Visio file with me, but I was not yet competent with MS Visio.  Well Sluckey said he did not have time to revise the layout and would get to it later, but instead he found the time and revised the layout and built the amp.  Well, I have one now as well, but a little better than the original due to the tube up design and some modifications.

Are you being sensitive?  Sensitive is like having a bad tooth.  It is usually ok until you hit it just right and I think you got hit just right.

BTW, I am not taking up for this dude.  The moderators here all have their strong points.  I am just sharing my experience.  I have not had to post questions for repairs in a long time nor have I posted voltages.  This is because I have learned to do this myself and do troubleshooting and actually beginning to understand all this wierd math.

I have been told "if it were wired correctly it would be working" which is actually true, but does not help much.  I have also been told a special power cord for my amp can help, which is not true. 

If you can and are still reading this, post your voltages and hang around long enough for someone to get drunk and really write what is on their mind.

And you asked "why should I endure negative comments"?  Well I can tell you that the $100 power cord Mercury Magnetics sells will NOT help your tone.  If you own one and are reading this you are pissed off right now.  Well I am sorry you are, but that is because your belief system has been disrupted and you are feeling dumb, but on your next build you can put that extra money on better speakers.

[highlux]

Well....good news.  Its working. Plays beautifully.    I am totally serious!   I will get a short video once I get the work area picked up.

Not a single thing mentioned here was the cause either.....my diligence and perseverance was what saved the day. 

[Ed_Chambley]

Well....good news.  Its working. Plays beautifully.    I am totally serious!   I will get a short video once I get the work area picked up.

Not a single thing mentioned here was the cause either.....my diligence and perseverance was what saved the day.

That is great, but of course nothing mentioned was the problem.  The reason I say that is with the info provided everyone was just guessing.

What exactly did you find to be the issue?

Also, hang around so you can help those who  know less than you and in a short while you too will say post your voltages.  I am in no way poking you, but with tube repair knowing voltages can sometimes immediately tell me what is the issue.

I am not good at looking a photos, but Silvergun is.  Others are too and I can see obvious things, but there is nothing like a meter closely followed by a scope.

Glad you got it going.   

[highlux]

Well....I thought it was a bad power tube.   I just happen to pull a couple 3l34eh out of a peavy classic 30...and i thought to myself...I know these are good....so I popped them in the amp.   Fired it up. Worked.     


BUT

I knew the owner had just bought the 2 new jj 3l34's that were in the amp. They were "new" and "matched".   

I couldn't accept it was just the tubes....so....you think most people would be ok...new tubes...it good to go now.  Then I remembered....those EH tubes are not mine...lol.
So I put the old jj's back in.  Put the EH's back in the Peavey.   Well....the amp still worked.

So...to be honest....I have no idea....what made the amp start working. Jiggling stuff?   Cleaning pots/jacks?

I think it was my mojo.   Karma fixed the amp despite professor grump amp.

I'm playing it on and off all evening to attempt to reproduce the issue.  So far so good.

Now to mop up the cord...and talk to owner about these 50 year old pre amp tubes and upgrading the speaker to a Celestion.

[Paul1453]

Some DeoxIT in the tube sockets should keep it running now.   

[highlux]

Some DeoxIT in the tube sockets should keep it running now.   

Did that first day.

Here is a little video...I'm a terrible guitar player...but I keep trying...lol

Very little clean headroom.  breaks up quick, like 9am on the dial.

[Ed_Chambley]

What power tubes are in the amp?  You wrote 3l34.  Is it EL84?  The amp sounds fine, but have you checked to see how old the filter caps are?  I mean, a general service on the amp before returning it to the owner is what I usually will do.

That is sort of like making sure all contacts are clean, like burnishing the input jack(s) if switchcraft, I will remove the pots and clean oxidation and make sure you have good contact.  I usually will replace plate resistors and bypass caps if old and electrolytic.  Maybe measure the old ones to see if they have drifted by drying up.  You know, a general service.  Checking solder joints and re-flowing any questionable joints.

Doing these things are not absolutely necessary, but if someone gave you the amp to repair these things will usually make the amp quieter and tighten up the bottom end.

Who is the amp tech you refer to as Karma.